JP2022098766A - Polyacrylonitrile fiber and method of producing the same - Google Patents

Abstract

To provide a polyacrylonitrile fiber which curbs formation of a void in a surface layer of a carbon fiber monofilament due to permeation of an oil agent to the surface layer of a polyacrylonitrile fiber monofilament that is used as a precursor fiber of a carbon fiber and from which the carbon fiber is obtained, the carbon fiber being able to be formed into a carbon fiber composite material excellent in interlayer fracture toughness.SOLUTION: A method of producing a polyacrylonitrile fiber includes: a coagulation step of obtaining a coagulated fiber by spinning a dope including a polyacrylonitrile-based polymer into a coagulant liquid; and a hot water drawing step of obtaining a hot water drawing coagulated fiber by drawing the coagulated fiber in how water. A temperature and turbidity of the coagulant liquid in the coagulation step are, respectively, 10-20°C and 10.0 or lower, and a maximum temperature of the hot water in the hot water drawing step is 90°C or higher.SELECTED DRAWING: None

Description

The present invention relates to a polyacrylonitrile-based fiber used as a precursor fiber for obtaining high-performance and high-quality carbon fiber, and a method for producing the same.

Carbon fiber is attracting attention in many fields as a reinforcing fiber for composite materials. It is widely known that polyacrylonitrile-based fibers are used as precursor fibers for carbon fibers, and since these polyacrylonitrile-based carbon fibers are excellent in strength, elasticity, heat resistance, etc., they are widely used as reinforcing fibers for composite materials. It is used.

Polyacrylonitrile-based fibers are generally formed by spinning a spinning stock solution in which a polyacrylonitrile-based polymer is dissolved in a solvent by a wet spinning method or a dry-wet spinning method to form fibers, and then drawing, washing and drying and densifying. Obtained by doing. As the solvent, an inorganic solvent or an organic solvent is used, an aqueous solution such as an aqueous solution of zinc chloride is widely used as the inorganic solvent, and dimethylformamide, dimethylacetamide, dimethylsulfoxide and the like are widely used as the organic solvent.

Carbon fibers produced using polyacrylonitrile-based fibers as precursor fibers heat the polyacrylonitrile-based fibers to 200 to 300 ° C. in an oxidizing atmosphere to convert them into flame-resistant fibers, and then convert them into flame-resistant fibers at 1000 ° C. or higher in an inert atmosphere. It is obtained by undergoing carbonization (firing) by heating to the temperature of. In this process, adhesion between the single fibers may occur, in which case the fluff is significantly increased and the strength of the obtained carbon fibers is significantly reduced.

In order to solve this problem, it is common practice to apply an oil agent to the precursor fiber in the process of manufacturing the precursor fiber. An oil agent containing silicone as a main component is used as the oil agent, and it is desirable that the precursor fiber is sufficiently permeated into the fiber bundle so as to be uniformly applied to the surface of the single fiber. However, if this oil agent permeates too much into the surface layer of the single fiber, it causes defects such as chipping on the fiber surface when the oil agent that has permeated too much disappears in the firing process. In that case, it causes deterioration of the physical properties of the composite material using the obtained carbon fiber as a reinforcing material.

As a countermeasure for this, for example, in Patent Documents 1 and 2, it is possible to produce carbon fibers having no adhesion between single yarns by setting the degree of swelling of the water bath drawn yarn within a specific range and adjusting the oiling agent application conditions and the spinning conditions. It has been disclosed.

Japanese Unexamined Patent Publication No. 2014-163012 Japanese Unexamined Patent Publication No. 2008-308776

However, the techniques described in Patent Documents 1 and 2 do not take into consideration the problem of foreign matter such as polymer polymerization catalyst residue existing in the coagulation bath and the generation of defects on the surface of the single fiber due to the adjustment of spinning conditions, and the precursor. It was not possible to sufficiently suppress the permeation of the oil agent into the surface layer of the single fiber of the polyacrylonitrile-based fiber used as the fiber. Therefore, even if the methods described in Patent Documents 1 and 2 are used, it is not possible to obtain carbon fibers having no surface defects as carbon fibers obtained by firing the precursor fibers, and the carbon fibers can be obtained. The interlayer fracture toughness of the carbon fiber composite material was not satisfactory.

An object of the present invention is to suppress the formation of voids on the surface layer of a single fiber of a carbon fiber caused by the permeation of an oil agent into the surface layer of a single fiber of a polyacrylonitrile fiber used as a precursor fiber of the carbon fiber. .. An object of the present invention is to provide a polyacrylonitrile-based fiber capable of obtaining a carbon fiber having excellent interlayer fracture toughness when used as a carbon fiber composite material.

The present inventors have diligently studied a method for producing a polyacrylonitrile-based fiber used as a precursor fiber in order to suppress defects on the surface and surface layer of the carbon fiber, and control the fiber swelling degree of the yarn after hot water drawing. As a result, the permeation of the oil agent into the fiber surface layer can be suppressed, and the foreign matter contained in the dope such as the catalyst residue flowing out into the coagulation bath is coagulated while being continuously filtered to form the carbon fiber surface and surface layer. It was found that the defect formation in the above can be suppressed.

That is, the present invention is a polyacrylonitrile fiber made of a polyacrylonitrile polymer, characterized in that it has a nodular modulus of 15 to 20 GPa and a single yarn strength of 7.0 to 10.0 cN / dtex. It is a polyacrylonitrile fiber.

The present invention also has a coagulation step of obtaining coagulated fibers by spinning a spinning stock solution containing a polyacrylonitrile-based polymer into a coagulating liquid, and a hot water-stretched coagulated fiber by stretching the coagulated fibers in hot water. A method for producing a polyacrylonitrile-based fiber, which comprises a hot water stretching step of obtaining, wherein the temperature of the coagulating liquid in the coagulation step is 10 to 20 ° C. and the turbidity is 10.0 or less, and the hot water in the hot water stretching step is obtained. It is a method for producing a polyacrylonitrile-based fiber, characterized in that the maximum temperature is 90 ° C. or higher.

According to the present invention, it is possible to suppress the formation of voids on the surface layer of the single fiber of the carbon fiber due to the permeation of the oil agent into the surface layer of the single fiber of the polyacrylonitrile fiber used as the precursor fiber of the carbon fiber. .. Then, it is possible to provide a polyacrylonitrile-based fiber capable of obtaining a carbon fiber having excellent interlayer fracture toughness when used as a carbon fiber composite material.

Hereinafter, the present invention will be described in detail.

[Polyacrylonitrile polymer]
The polyacrylonitrile-based polymer constituting the polyacrylonitrile-based fiber of the present invention is a polymer containing acrylonitrile as a main monomer component, and the acrylonitrile component is preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 90% by mass or more. It is a polymer containing 95 to 99% by mass.

The polyacrylonitrile-based polymer preferably contains a comonomer component that can be copolymerized with acrylonitrile having a vinyl skeleton from the viewpoint of solubility in a solvent. As the comonomer component copolymerizable with acrylonitrile, a vinyl compound containing a carboxyl group is preferably used. For example, acrylic acid, itaconic acid and salts thereof, acrylic acid esters such as methyl acrylate and ethyl acrylate, and amides such as acrylamide can be exemplified. From the viewpoint of accelerating the flame resistance reaction, a carboxyl group-containing vinyl compound, particularly preferably itaconic acid, is used as the comonomer component copolymerizable with acrylonitrile.

The polyacrylonitrile-based polymer having a weight average molecular weight of preferably 100,000 or more, more preferably 150,000 to 1,000,000, and particularly preferably 200,000 to 800,000 is used. Further, the value of the weight average molecular weight / number average molecular weight (Mw / Mn) of the polyacrylonitrile-based polymer is preferably 2.0 or more, more preferably larger than 2. In order to adjust the value of Mw / Mn, polyacrylonitrile-based polymers having different molecular weight distributions may be mixed and used.

The polyacrylonitrile-based polymer may be polymerized by any of a water-based suspension polymerization method and a solution polymerization method. When a solution polymerization method is used as the polymerization method, dimethyl sulfoxide is preferable as the solvent.

[Nodular modulus]
The nodular elastic modulus of the polyacrylonitrile-based fiber of the present invention is 15 GPa to 20 GPa, preferably 16 to 19 GPa. When the nodular elastic modulus is less than 15 GPa, the strand tensile strength of the obtained carbon fiber is lowered. On the other hand, when the knot elastic modulus exceeds 20 GPa, the interlayer fracture toughness (G1c) of the carbon fiber composite material using the obtained carbon fiber as a reinforcing material is lowered.

[Single yarn strength]
The single yarn strength of the polyacrylonitrile-based fiber of the present invention is 7.0 to 10.0 cN / dtex, preferably 8.0 to 10.0 cN / dtex. When the single yarn strength is less than 7.0 cN / dtex, the fiber is easily cut in the carbon fiber manufacturing process, and the quality of the obtained carbon fiber is deteriorated. On the other hand, when the single yarn strength exceeds 10.0 cN / dtex, the strand tensile strength of the obtained carbon fiber is rather lowered.

[Manufacturing method of polyacrylonitrile fiber]
The method for producing a polyacrylonitrile fiber of the present invention is a coagulation step of obtaining a coagulated fiber by spinning a spinning stock solution containing a polyacrylonitrile polymer into a coagulating solution, and stretching the coagulated fiber in hot water. It is provided with a hot water stretching step of obtaining a hot water stretched coagulated fiber.

[Spinning stock solution]
The concentration of the polyacrylonitrile-based polymer in the spinning stock solution is preferably 10 to 30% by weight, more preferably 15 to 25% by weight. By setting the concentration in this range, carbon fiber precursor fibers can be obtained with excellent productivity. The solvent for dissolving the polyacrylonitrile-based polymer in the spinning stock solution is preferably dimethyl sulfoxide.

After preparation, the undiluted spinning solution is passed through a filter medium to filter out gel-like foreign substances and insoluble components, and is used in the spinning process. From the viewpoint of productivity and performance, it is preferable that the spinning solution is highly defoamed or defoamed and has no bubbles. Defoaming or defoaming can be accelerated by pressurization or depressurization.

The undiluted spinning solution may contain additives. In particular, from the viewpoint of controlling coagulation, it is preferable to add a compound that neutralizes the acid component remaining in the solution of the polyacrylonitrile-based copolymer. As this compound, a basic compound is used, and a non-metal compound is preferably used, particularly preferably ammonia, from the viewpoint of suppressing defects in the carbon fiber precursor fiber.

[Spinning]
The undiluted spinning solution containing the polyacrylic nitrile polymer prepared as described above is spun and coagulated in the coagulating solution to become coagulated fibers. When spinning a spinning stock solution containing a polyacrylonitrile-based polymer and coagulating it in a coagulating liquid, it is possible to adopt a dry-wet spinning method in which the spinning stock solution is once extruded into the air and then infiltrated into a coagulation bath. It is preferable because the surface layer of the polyacrylonitrile-based fiber becomes relatively smooth and the interlayer fracture toughness when used as a carbon fiber composite material is easily improved. On the other hand, if a completely wet spinning method that does not pass through the air is adopted without adopting this method, wrinkles are formed on the surface layer of the polyacrylonitrile fiber, and the fibers are formed into a carbon fiber composite material due to the anchor effect of the wrinkles. While the adhesive strength of the matrix resin is improved, the interlayer fracture toughness may be lowered.

The spinneret for spinning the undiluted spinning solution has, for example, 100 to 100,000, preferably 1000 to 80,000, and more preferably 3000 to 50,000 discharge holes. The hole diameter of the discharge hole of the spinneret is preferably 0.02 to 0.5 mm. When the pore diameter is 0.02 mm or more, the discharged threads are less likely to adhere to each other, so that an acrylonitrile-based fiber having excellent homogeneity can be obtained. When the hole diameter is 0.5 mm or less, the occurrence of spinning yarn breakage can be suppressed and the spinning stability can be maintained.

[Coagulant]
The temperature of the coagulating liquid in the coagulation step is 10 to 20 ° C, preferably 10 to 15 ° C. If the temperature of the coagulating liquid is less than 10 ° C., adhesion between the single yarns of the carbon fibers tends to occur, and if the temperature of the coagulating liquid exceeds 20 ° C., the cross section of the polyacrylonitrile fiber tends to be elliptical, which is not preferable. It is necessary to keep the temperature of the coagulating liquid at 10 to 20 ° C. from the viewpoint of suppressing the adhesion between the single fibers of the obtained carbon fibers.

The turbidity of the coagulation liquid in the coagulation step is 10.0 or less, preferably 5.0 or less, and more preferably 1.0 or less. When the turbidity of the coagulation liquid exceeds 10.0, defects on the surface of the finally obtained carbon fiber become remarkable, and it becomes difficult to obtain a high knot elastic modulus of the polyacrylonitrile-based fiber. From the viewpoint of suppressing surface defects of the obtained carbon fiber, it is necessary to keep the turbidity of the coagulation liquid at 10.0 or less.

[Precision filter]
The coagulation liquid in the coagulation bath is preferably continuously filtered by a precision filter. If this filtration is not performed, the turbidity of the coagulation bath may become too high, and the coagulation fibers tend to have surface defects. This is because foreign substances existing in the surrounding atmosphere of the process and foreign substances such as polymerization catalyst residues originally contained in the undiluted spinning solution migrate to the coagulating solution in the coagulation bath and adhere to the surface or surface layer of the coagulating fibers. Is. This attached foreign matter causes surface defects when the polyacrylonitrile-based polymer is made into carbon fiber.

The precision filter is preferably installed in a coagulation liquid circulation line connected to a coagulation bath containing the coagulation liquid. By providing a coagulation liquid circulation line that continuously circulates the coagulation liquid, the coagulation liquid can be continuously filtered. The precision filter is preferably installed in a coagulation liquid circulation line connected to a coagulation bath containing the coagulation liquid.

The filtration accuracy of this precision filter is preferably 0.1 to 30 μm, more preferably 0.1 μm to 10 μm. If the filtration accuracy is less than 0.1 μm, the pressure loss becomes too high, which is not preferable. On the other hand, if it exceeds 30 μm, foreign matter cannot be sufficiently filtered, which may cause defects on the surface of the carbon fiber, which is not preferable.

[Coagulant]
As the coagulation liquid used in the coagulation bath, an aqueous solution or an organic solvent aqueous solution can be used. For example, zinc chloride aqueous solution, dimethylacetamide, dimethyl sulfoxide, dimethylformamide can be mentioned.

In the present invention, it is important that the coagulation liquid in the coagulation bath is continuously filtered by the precision filter as described above. The circulating flow rate of the coagulant is preferably 5 L / min or more. If the circulation flow rate is less than 5 L / min, it may not be possible to sufficiently filter foreign substances that enter the coagulant.

[Hot water stretching]
The coagulated fibers of the polyacrylonitrile-based polymer obtained by coagulating in the coagulating liquid are washed with water and stretched in hot water. It is preferable to wash with water at a temperature of 90 ° C. or higher in the water washing bath.

The maximum temperature of hot water in the hot water stretching step is 90 ° C. or higher. If the maximum temperature of hot water is less than 90 ° C., the tension applied to the coagulated yarn in the hot water drawing step becomes high, and the tension is concentrated on the surface of the coagulated yarn where defects are likely to occur, and the surface of the obtained polyacrylonitrile fiber. Defects increase. Since the hot water stretching bath used for hot water stretching has a certain size, the temperature of the hot water may differ depending on the position due to the temperature difference between the room temperature and the hot water. The maximum temperature of hot water is the temperature of hot water at the highest temperature position in the hot water stretching bath used in the hot water stretching step.

[Fiber swelling degree]
The fiber swelling degree of the coagulated fiber after hot water stretching is preferably 80 to 110%, more preferably 80 to 100%, and particularly preferably 85 to 100%. Within this range, the oil agent can be uniformly applied to the inside of the bundle of fibers, and the permeation of the oil agent into the fiber surface layer can be suppressed. When the fiber swelling degree of the coagulated fiber after hot water stretching is less than 80%, the oil agent does not penetrate into the fiber bundle when the oil agent is applied, and a part where the oil agent is not applied is formed, and the single fiber is formed. There is a tendency for the close contact between them to increase. On the other hand, if the fiber swelling degree of the coagulated fiber after hot water stretching exceeds 110%, the oil agent may permeate into the fiber surface layer and cause surface void formation in the firing step. The degree of fiber swelling can be adjusted to a desired value by adjusting the molecular weight, polymer concentration, coagulation liquid temperature, concentration, hot water stretching temperature, and magnification of the polyacrylonitrile-based polymer used.

[Adding oil]
The coagulated fibers are hot-water-stretched after the coagulation step, and it is preferable that an oil agent is further applied to the stretched coagulated fibers after the hot-water-stretched. As a method of applying the oil agent, a method of immersing the threads of the drawn coagulated fibers after hot water stretching in an aqueous solution containing the oil agent to bring the surface of the drawn coagulated fibers into contact with the oil agent can be used. A silicone-based oil is preferably used as the oil from the viewpoint of preventing adhesion between the single fibers of the stretched coagulated fibers and improving heat resistance, releasability, and process passability.

Examples of the silicone-based oil agent include amino-modified silicone, epoxy-modified silicone, and ether-modified silicone. These may be used by mixing two or more kinds. The amount of the oil adhered is preferably 0.01 to 10.0% by weight, more preferably 0.1 to 5.0% by weight, still more preferably 0.1 to 1.0% based on the weight of the drawn coagulated fiber. Is. By setting the amount of oil adhering to this range, it is possible to efficiently suppress the occurrence of yarn breakage and fluff in the subsequent flame resistance step, and to obtain high quality carbon fiber.

If the amount of the oil adhered is less than this, the oil does not sufficiently adhere to the surface of the stretched coagulated fiber, so that thread breakage and fluffing are likely to occur in the flame resistance process, while the amount of the oil adhered is more than this. If the amount is too large, the oil agent will be deposited on the surface of the fiber transport roller or the guide in the flame resistance process, which is likely to cause yarn breakage, which is not preferable.

[Drying and densification]
It is preferable that the stretched coagulated fiber to which the oil agent is applied is subjected to a drying and densifying treatment with a dry heat roller. The temperature of the dry heat roller is preferably 150 ° C. or higher, more preferably 150 ° C. to 230 ° C., still more preferably 160 ° C. to 220 ° C., and particularly preferably 160 ° C. to 200 ° C.

If the temperature of the dry heat roller in the dry densification treatment is less than 150 ° C., the densification of the stretched coagulated fibers becomes insufficient, and the stretchability tends to decrease in the subsequent steam stretching step. On the other hand, if the drying densification temperature exceeds 230 ° C., the flame resistance reaction may proceed at the stage of drying densification.

[Post-stretching]
The stretched coagulated fibers after the dry densification treatment with a dry heat roller are preferably further subjected to a post-stretching treatment. The stretching method for the post-stretching treatment is preferably steam stretching. In this case, the saturated steam pressure for steam stretching is preferably 0.15 to 0.8 MPa.

The stretching ratio in steam stretching is preferably 2 to 10 times, more preferably 2 to 5 times, and particularly preferably 2.0 to 3.0 times. The temperature of steam stretching is preferably 105 to 200 ° C, more preferably 110 to 180 ° C.

The draw ratio is preferably 10 to 20 times, more preferably 10 to 17 times, as the total draw ratio through hot water stretching immediately after spinning, drying and post-stretching treatment. The fineness of the polyacrylonitrile fiber after steam stretching is preferably 0.5 to 1.7 dtex.

[Manufacturing method of carbon fiber]
Next, a method for producing carbon fiber using the polyacrylonitrile-based fiber of the present invention as a precursor fiber will be described.
The polyacrylonitrile fiber of the present invention undergoes a flame resistance step of firing in an oxidizing atmosphere at a temperature of 200 to 300 ° C. and a carbonization step of firing in an inert atmosphere at a temperature of 1000 ° C. or higher. To make carbon fiber.

[Preliminary heat treatment]
In the method for producing carbon fiber using the polyacrylonitrile-based fiber of the present invention, it is preferable to perform a preliminary heat treatment (preliminary flame resistant treatment) on the precursor fiber before the flame resistant treatment. This preheat treatment is preferably carried out at a temperature of 200 to 260 ° C., preferably at a stretching ratio of 0.80 to 1.20.

[Flame resistant treatment]
The precursor fiber that has been preheat-treated (pre-flame resistant treatment) is subsequently subjected to flame resistant treatment at 200 to 300 ° C., more preferably 200 to 260 ° C. in heated air. The flame resistance treatment is preferably performed while the stretching treatment is performed in the range of a stretching ratio of 0.85 to 1.15, and the stretching ratio is 0.90 to 1 in order to obtain carbon fibers having high strength and high elastic modulus. .15 is preferred. This flame-resistant treatment is preferably performed until the precursor fiber becomes a flame-resistant fiber which is an oxidized polyacrylonitrile-based fiber having a fiber density of 1.34 to 1.38 g / cm ³ .

[Carbonization treatment]
Carbon fibers can be produced by carbonizing the flame-resistant fibers obtained above. The carbonization treatment is carried out at a temperature of 1000 ° C. or higher in an inert atmosphere.
In the present invention, the carbonization treatment is preferably carried out by first carbonizing the flame-resistant fiber in an inert atmosphere at a temperature of less than 1000 ° C. and then performing a second carbonization treatment at a temperature of 1000 ° C. or higher. ..

In the first carbonization treatment, it is preferable to gradually raise the temperature in a first carbonization furnace at 300 to 800 ° C. under a nitrogen atmosphere and control the tension of the flame-resistant fiber. The maximum temperature of the first carbonization furnace is preferably 550 to 700 ° C, more preferably 620 ° C or higher. The minimum temperature (inlet temperature) of the first carbonization furnace is preferably 300 to 500 ° C, more preferably 300 to 450 ° C. The residence time of the first carbonization furnace is preferably 1 minute or more, more preferably 2 to 20 minutes.

In the second carbonization treatment, in order to further promote carbonization and graphitization (high crystallization of carbon), gradually in a second carbonization furnace at 500 to 1800 ° C. under an inert gas atmosphere such as nitrogen. It is preferable to raise the temperature and control the tension for firing.

In each carbonization furnace, it is preferable to gradually change the temperature from the vicinity of the inlet of the furnace because it is easy to suppress surface defects and internal defects. In the above-mentioned first carbonization treatment and second carbonization treatment, the treatment may be performed using a plurality of furnaces.

The inlet temperature of the second carbonization furnace is preferably 550 to 700 ° C, more preferably 600 to 650 ° C. The maximum temperature of the second carbonization furnace is preferably 1400 to 1750 ° C, more preferably 1500 to 1700 ° C. The residence time in the second carbonization furnace is preferably 2.5 minutes or more, more preferably 2.5 to 10 minutes.
In the second carbonization furnace, the carbonization treatment is carried out while applying a tension of preferably 160 to 300 mg / dtex, more preferably 180 to 250 mg / dtex.

The temperature gradient through the total carbonization step, which is a combination of the first carbonization treatment in the first carbonization furnace and the second carbonization treatment in the second carbonization furnace, is preferably 300 to 600 ° C./min, more preferably. Is 300 to 450 ° C./min.

[Surface oxidation treatment]
It is preferable that the carbon fibers obtained through the carbonization treatment in this manner are subsequently subjected to surface oxidation treatment in the electrolytic solution. The amount of electricity processed at this time is preferably 10 to 250 C / g, and more preferably 20 to 200 C / g. The larger the amount of electricity processed, the larger the surface oxygen concentration ratio O / C of the carbon fiber, and the adhesiveness between the carbon fiber and the matrix resin tends to improve. However, if the amount of electricity processed is too large, the surface of the carbon fiber May have defects.

As the electrolytic solution, an aqueous solution of an inorganic acid such as nitric acid or sulfuric acid or an aqueous solution of an inorganic acid salt such as ammonium sulfate can be used, and an aqueous solution of ammonium sulfate is preferable from the viewpoint of safety and handleability. The temperature of the electrolytic solution is preferably 20 to 50 ° C., and the concentration of the electrolytic solution is preferably 0.5 to 2.0 N, more preferably 0.7 to 1.5 N.

[Sizing agent treatment]
It is preferable that the surface-oxidized carbon fibers are passed through a sizing solution in the form of carbon fiber bundles to be imparted with a sizing agent. The concentration of the sizing agent in the sizing solution is preferably 10 to 25% by mass, and the amount of the sizing agent adhered is preferably 0.4 to 1.7% by mass.

Examples of the sizing agent applied to the carbon fiber bundle include epoxy resin, urethane resin, polyester resin, vinyl ester resin, polyamide resin, polyether resin, acrylic resin, polyolefin resin, polyimide resin and modified products thereof. .. A suitable sizing agent can be appropriately selected depending on the matrix resin of the composite material. Two or more types of sizing agents may be used in combination.

As the sizing agent applying treatment, an emulsion method in which a carbon fiber bundle is immersed in an aqueous emulsion obtained by using an emulsifier or the like can be used. Auxiliary components such as a dispersant and a surfactant may be added to the sizing agent in order to improve the handleability, scratch resistance, fluff resistance and impregnation property of the carbon fiber.

The carbon fiber bundle after the sizing agent application treatment is subjected to a drying treatment for evaporating the dispersion medium of the sizing agent, and carbon fibers are obtained in the form of the carbon fiber bundle. It is preferable to use an air dryer for the drying treatment. The temperature of the drying treatment is, for example, 100 to 180 ° C. in the case of a general-purpose aqueous emulsion. After the drying treatment, further heat treatment may be performed at a temperature of 200 ° C. or higher.

Hereinafter, the present invention will be specifically described with reference to examples and the like. Various evaluations in Examples and Comparative Examples were carried out by the following methods.

(1) Fiber swelling degree Approximately 1 g of coagulated fiber after hot water stretching was sampled, washed with water for 12 hours, and dehydrated at 3000 rpm for 3 minutes with a stretching dehydrator (manufactured by H-Kokusan Co., Ltd.) to determine the fiber weight after dehydration. rice field. The dehydrated sample was dried for 2 hours in a hot air dryer whose temperature was adjusted to 120 ° C., the weight of the dried fiber was determined, and the degree of fiber swelling was calculated by the following formula.
Fiber swelling degree (%)
= ((Fiber weight after dehydration-Fiber weight after drying) / Fiber weight after drying)) × 100

(2) Turbidity of coagulant The turbidity of the coagulant was measured with a turbidity meter (manufactured by TR-55 AS ONE Co., Ltd.).

(3) Knot elastic modulus The measurement was carried out with a tensile tester (manufactured by Shimadzu Corporation, RTG Co., Ltd.) under the conditions of a test length of 250 mm and a test speed of 300 mm / min.

(4) Single yarn strength A single fiber was measured with a single fiber tensile tester (manufactured by Shimadzu Corporation, EZ-SX) under the conditions of a test length of 25 mm and a test speed of 20 mm / min.

(5) Surface void ratio A thin film sample was prepared with a cryoion slicer (EM-09100IS, manufactured by JEOL Ltd.) under the condition of −120 ° C. The obtained thin film sample was observed with a transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) at an acceleration voltage of 200 kV. For 100 single carbon fibers, voids were manually extracted from the transmission electron microscope image in the range from the surface layer to 100 nm, and the area of the voids and the area of the measurement area were calculated using the image processing software Nexus Newcube. The area of the void / the area of the measurement area was calculated and used as the surface void ratio.

(6) Surface Defects From the transmission electron microscope image taken in (5) above, the number of defects with a depth of 20 nm or more from the surface of 100 carbon fiber single fibers is counted, and the number of defects per fiber is counted. Was calculated.

(7) Interlayer fracture toughness (G1c)
A prepreg for evaluation was prepared and evaluated using the following resin composition.

<Resin composition>
The following were prepared as the components of the resin composition.
(Epoxy resin)
MY0600: glycidylamine type epoxy resin, Araldite MY0600 (trade name) manufactured by Huntsman Advanced Materials, 35 parts by weight ・ EP604: glycidylamine type epoxy resin, EP604 (trade name) manufactured by Japan Epoxy Resin, 30 parts by weight ・ EP828 : Bisphenol A type epoxy resin, EP828 (trade name) manufactured by Japan Epoxy Resin, 15 parts by weight ・ EPU-6: Urethane-modified epoxy resin, EPU-6 (trade name) manufactured by Adeca, 20 parts by weight (curing agent)
-Aromatic amine-based curing agent: 4,4'-diaminodiphenyl sulfone (Seika Cure S (trade name) manufactured by Wakayama Seika Kogyo Co., Ltd.) 40 parts by weight (polyether sulfone)
-Polyether sulfone: PES-5003P (trade name) manufactured by Sumitomo Chemical Co., Ltd. 35 parts by weight (polyamide resin particles)
-Polyamide resin particles: Grillamide TR-55 (trade name) manufactured by Ems-Chemie Japan Co., Ltd. 20 parts by weight

<Preparation of resin composition>
A polyether sulfone was added to the above epoxy resin, and the mixture was stirred at 120 ° C. for 60 minutes using a planetary mixer to completely dissolve the polyether sulfone in the epoxy resin. After cooling the resin temperature to 80 ° C. or lower, polyamide resin particles and a curing agent were added and kneaded using a roll mill to prepare an epoxy resin composition.

<Manufacturing of prepreg>
The prepared epoxy resin composition was applied onto each release paper using a film coater to prepare two 50 g / m ² resin films. Next, one resin film produced above was laminated on both sides of the carbon fiber sheet in which the carbon fiber bundles were arranged in one direction. By heating and pressurizing, the resin of the resin film was impregnated into the carbon fiber sheet to prepare a unidirectional prepreg having a carbon fiber texture of 190 g / m ² and a mass fraction of the matrix resin of 35.0%.

<Preparation and test method of measurement sample>
The unidirectional prepreg was cut into a square having a side of 360 mm and then laminated to prepare two laminated bodies in which 10 layers were laminated in the 0 ° direction. In order to generate initial cracks, a release sheet was sandwiched between two laminated bodies, and both were combined to obtain a prepreg laminated body having a laminated structure [0] ₂₀ . Using a normal vacuum autoclave molding method, molding was performed under a pressure of 0.59 MPa at 180 ° C. for 2 hours. The obtained molded product (fiber-reinforced composite material) was cut to a size of 12.7 mm in width × 304.8 mm in length to obtain a test piece of interlayer fracture toughness mode I (GIc). As a GIc test method, a double cantilever beam interlayer fracture toughness test method (DCB method) is used to generate a pre-crack (initial crack) of 12.7 mm from the tip of the release sheet, and then further develop the crack. Was done. The test was terminated when the crack extension length reached 127 mm from the tip of the pre-crack. The crosshead speed of the test piece tensile tester was 12.7 mm / min, and the measurement was performed at n = 5. The crack growth length was measured from both end faces of the test piece using a microscope, and the interlaminar fracture toughness (G1c) was calculated by the integral method by measuring the load and the crack opening displacement.

(8) Carbon fiber adhesion number (CF adhesion number)
The carbon fiber bundles were combined so that the number of carbon fiber single fibers was 24,000, and the strands cut to a length of 2 mm were ultrasonically treated in a beaker containing ethanol for 1 minute. ) Using Kaijo's desktop ultrasonic cleaner Sono Cleaner 200D), ethanol was transferred to a chalet, and the number of places where single yarns were in the form of two or more lumps was measured with a stereoscopic microscope. The measurement was performed 5 times, and the measurement result was the average value. The measurement results are expressed by the number of contacts (pieces / 24K) per 24,000 samples with a length of 2 mm.

(9) Strand tensile strength of carbon fiber (CF strength)
The strand tensile strength (CF strength) of the carbon fiber was measured by the method specified in JIS R 7608.

[Example 1]
354 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile and 1 part by weight of itaconic acid were charged in a polymerization tank having a stirring blade in the tank, and the mixture was stirred and mixed so as to be uniform, and the temperature was raised to 60 ° C. After reaching 60 ° C., 0.4 part by weight of azobisisobutyronitrile and 0.1 part by weight of octyl mercaptan were added to start the reaction. The temperature was controlled so that the reaction temperature was 60 ° C. up to 4 hours after the start of the reaction. Then, the temperature was raised for 2 hours at a rate of 10 ° C./hour. For the following 6 hours, the temperature was controlled so that the reaction temperature was 80 ° C. to obtain a dope. The unreacted acrylonitrile was distilled off by reducing the pressure of the obtained dope. Subsequently, ammonia gas was blown into the dope, and the mixture was stirred and mixed so as to be uniform, to obtain a spinning stock solution of a polyacrylonitrile-based polymer. The obtained undiluted spinning solution was transferred to a storage tank.

This undiluted spinning solution is discharged from a spinning spout having a pore diameter of 150 μm and a number of holes of 3000, the distance between the spinning spout and the coagulating liquid surface is set to 5 mm, and the mixture is coagulated in the coagulating liquid to perform dry-wet spinning. Combined coagulated fibers were obtained. The temperature of the coagulation liquid in the coagulation bath was 15 ° C., and the coagulation liquid was continuously filtered with a precision filter having a filtration accuracy of 0.5 μm to maintain the turbidity of the coagulation liquid at 0.1. The obtained coagulated fibers were desolvated in a water washing tank and stretched 3.0 times in a hot water stretching bath having a hot water temperature of 98 ° C. at the highest temperature. The degree of swelling of the coagulated fibers after hot water stretching was 87%. The precision filter used for filtering the coagulating liquid is installed in the coagulating liquid circulation line connected to the coagulating bath containing the coagulating liquid.

The coagulated fibers after stretching with hot water are immersed in a silicone-based oil bath to apply an oil, dried and densified at 160 ° C for 30 seconds with a heating roller, and after 4.0 times in steam at a pressure of 0.40 MPa. The fiber was stretched and finally heat-fixed at 140 ° C. for 10 seconds to obtain a fiber bundle of polyacrylonitrile-based fibers. The fineness of the single fiber of the fiber bundle of the obtained polyacrylonitrile fiber was 0.8 dtex / fill, the nodular elastic modulus was 19.0 GPa, and the single fiber strength was 8.2 cN / dtex.

Two of the obtained polyacrylonitrile-based fiber bundles were combined to make the total number of filaments 6000, and then flame-resistant in air at a temperature of 240 to 260 ° C., followed by a flame resistance treatment at 300 to 700 ° C. Preliminary carbonization treatment was carried out in a nitrogen atmosphere at a temperature, and carbonization treatment was further carried out in a nitrogen atmosphere at a maximum temperature of 1500 ° C. to obtain carbon fiber bundles. The surface void ratio of the single fiber of the obtained carbon fiber bundle was 0.02%, no surface defects were observed, the CF strength was 6700 MPa, the interlaminar fracture toughness (G1c) was 796 J / m ² , and the number of CF adhesions was 5 /. It was 24K and showed good performance.

[Example 2]
Polyacrylonitrile-based fibers and carbon fibers were obtained in the same manner as in Example 1 except that the molecular weight distribution Mw / Mn of the polyacrylonitrile-based polymer was changed from 2.2 to 2.8 in Example 1.

The swelling degree of the coagulated fiber after hot water stretching was 90%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 19.0 GPa, and the single fiber strength was 8.8 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 0.00%, no surface defects were observed, the CF strength was 6600 MPa, the interlaminar fracture toughness (G1c) was 675 J / m ² , and the number of CF adhesions was 5 / 24K. It showed good performance.

[Example 3]
The polyacrylonitrile fiber and the polyacrylonitrile fiber and the same as in Example 1 except that the temperature of the coagulating liquid in the coagulation bath was changed from 15 ° C. to 10 ° C. Obtained carbon fiber. The swelling degree of the coagulated fiber after hot water stretching was 90%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 19.5 GPa, and the single fiber strength was 9.3 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 0.02%, no surface defects were observed, the CF strength was 6300 MPa, the interlaminar fracture toughness (G1c) was 571 J / m ² , and the number of CF adhesions was 5 / 24K. It showed good performance.

[Example 4]
Polyacrylonitrile-based fibers and carbon fibers were obtained in the same manner as in Example 2 except that the temperature of the coagulating liquid in the coagulation bath was changed from 15 ° C to 20 ° C in Example 2. The degree of swelling of the coagulated yarn after hot water drawing was 98%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 18.0 GPa, and the single fiber strength was 7.8 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 0.33%, no surface defects were observed, the CF strength was 6100 MPa, the interlaminar fracture toughness (G1c) was 588 J / m ² , and the number of CF adhesions was 1 / 24K. It showed good performance.

[Example 5]
In Example 2, the temperature of the coagulating liquid in the coagulation bath was changed from 15 ° C. to 20 ° C., and the fineness of the polyacrylonitrile fiber was changed to 1.3 dtex / fill, but the polyacrylonitrile was changed in the same manner as in Example 2. System fibers and carbon fibers were obtained. The swelling degree of the hot water drawn yarn was 85%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 17.0 GPa, and the single fiber strength was 7.0 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 0.00%, no surface defects were observed, the CF strength was 5400 MPa, the interlaminar fracture toughness (G1c) was 623 J / m ² , and the number of CF adhesions was 2 / 24K. It showed good performance.

[Example 6]
Polyacrylonitrile fibers and carbon fibers were obtained in the same manner as in Example 2 except that the filtration accuracy of the coagulation bath filter was changed from 0.5 μm to 30 μm in Example 2. The swelling degree of the hot water drawn yarn was 90%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 19.0 GPa, and the single fiber strength was 8.5 cN / dtex. The surface void ratio of the obtained single carbon fiber was 0.03%, no surface defects were observed, the CF strength was 6500 MPa, the interlaminar fracture toughness (G1c) was 571 J / m ² , and the number of CF adhesions was 3 /. It was 24K and showed good performance.

[Comparative Example 1]
Polyacrylonitrile-based fibers and carbon fibers were obtained in the same manner as in Example 2 except that the coagulation bath filter was not used in Example 2. The degree of swelling of the hot water drawn yarn was 90%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 20.5 GPa, and the single fiber strength was 9.0 cN / dtex. The surface void ratio of the obtained single carbon fiber was 0.01%, the CF strength was 6500 MPa, and the number of CF adhesions was 4 / 24K, which were good performance, but 21 surface defects / piece. It was present and the interlayer fracture toughness (G1c) remained at 415 J / m ² .

[Comparative Example 2]
Polyacrylonitrile fibers and carbon fibers were obtained in the same manner as in Example 1 except that the temperature of the coagulating liquid in the coagulation bath was changed to 3 ° C. in Example 1. The swelling degree of the hot water drawn yarn was 70%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 22.0 GPa, and the single fiber strength was 9.4 cN / dtex. The surface void ratio of the obtained single carbon fiber was 0.00%, no surface defects were observed, the CF strength was 6300 MPa, the interlaminar fracture toughness (G1c) was 588 J / m ² , and the number of CF adhesions was 40 /. It was 24K, and the CF adhesion number was insufficient for the purpose.

[Comparative Example 3]
In Example 5, the temperature of the coagulating liquid in the coagulation bath was changed from 15 ° C to 3 ° C, the maximum temperature of the hot water stretching bath was changed to 70 ° C, and the fineness of the polyacrylonitrile fiber was changed to 1.3 dtex / fill. Polyacrylonitrile-based fibers and carbon fibers were obtained in the same manner as in Example 5 except for the above. The swelling degree of the hot water drawn yarn was 120%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 21.0 GPa, and the single fiber strength was 9.3 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 10.54%, 10 surface defects / line were present, the CF strength was 5000 MPa, the interlaminar fracture toughness (G1c) was 294 J / m ² , and the CF adhesion number was. The number was 5 / 24K, and the CF strength and G1c were insufficient for the purpose.

[Comparative Example 4]
Polyacrylonitrile-based fibers and carbon fibers were obtained in the same manner as in Example 2 except that the maximum temperature of the hot water stretching bath was changed to 80 ° C. in Example 2. The swelling degree of the hot water drawn yarn was 114%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 19.0 GPa, and the single fiber strength was 10.6 cN / dtex. The surface void ratio of the obtained single fiber of the carbon fiber was 2.50%, 13 surface defects / line were present, the CF strength was 6000 MPa, the interlaminar fracture toughness (G1c) was 415 J / m ² , and the number of CF adhesions. Was 7 / 24K, which was an insufficient result for the purpose of GIc.

[Comparative Example 5]
Polyacrylonitrile fibers and carbon fibers were obtained in the same manner as in Comparative Example 2 except that the maximum temperature of the hot water stretching bath was changed to 80 ° C. in Comparative Example 2. The swelling degree of the hot water drawn yarn was 93%, the knot elastic modulus of the obtained polyacrylonitrile fiber was 22.0 GPa, and the single fiber strength was 10.5 cN / dtex. The surface void ratio of the obtained carbon fiber single fiber was 0.03%, 8 surface defects / line were present, the CF strength was 6200 MPa, the interlaminar fracture toughness (G1c) was 502 J / m ² , and the CF adhesion number was. It was 4 pieces / 24K, and the result was insufficient for the purpose of GIc.

The polyacrylonitrile-based fiber of the present invention can be used as a precursor fiber of carbon fiber. The carbon fiber obtained from the polyacrylonitrile-based fiber of the present invention can be used as a reinforcing fiber of a fiber-reinforced composite material.

Claims (10)

A polyacrylonitrile fiber made of a polyacrylonitrile polymer, characterized by a knot elastic modulus of 15 to 20 GPa and a single yarn strength of 7.0 to 10.0 cN / dtex. fiber. The polyacrylonitrile-based fiber according to claim 1, which is used as a precursor fiber for carbon fiber. A coagulation step of obtaining coagulated fibers by spinning a spinning stock solution containing a polyacrylonitrile-based polymer into a coagulating liquid, and a hot water stretching step of stretching the coagulated fibers in hot water to obtain hot water-stretched coagulated fibers. A method for producing a polyacrylonitrile fiber having A method for producing a polyacrylonitrile-based fiber, which is characterized by the above. The polyacrylonitrile system according to claim 3, wherein in the coagulation step, when the undiluted spinning solution is spun into the coagulation solution, the spinning solution is extruded into the air and then infiltrated into the coagulation solution to be spun into the coagulation solution. Fiber manufacturing method. The method for producing a polyacrylonitrile-based fiber according to claim 3 or 4, wherein the coagulated fiber has a fiber swelling degree of 80 to 110% after hot water stretching. The method for producing a polyacrylonitrile-based fiber according to any one of claims 3 to 5, wherein the coagulating liquid in the coagulation step is continuously filtered by a precision filter. The method for producing a polyacrylonitrile-based fiber according to claim 6, wherein the filtration accuracy of the precision filter is 0.1 to 30 μm. The method for producing a polyacrylonitrile-based fiber according to any one of claims 3 to 7, wherein the precision filter is installed in a coagulation liquid circulation line connected to a coagulation bath containing the coagulation liquid. A step of obtaining flame-resistant fibers by flame-resistant the polyacrylonitrile-based fiber according to claim 1 or 2 at a temperature of 200 to 300 ° C. in an oxidizing atmosphere, and 1000 of the flame-resistant fibers in an inert atmosphere. A method for producing carbon fiber, which comprises a step of obtaining carbon fiber by carbonizing at a temperature of ° C. or higher. Flame resistant by making the polyacrylonitrile fiber obtained by the method for producing a polyacrylonitrile fiber obtained by the production method according to any one of claims 3 to 8 flame resistant at a temperature of 200 to 300 ° C. in an oxidizing atmosphere. A method for producing carbon fiber, comprising a step of obtaining chemical fiber and a step of obtaining carbon fiber by carbonizing the flame-resistant fiber at a temperature of 1000 ° C. or higher in an inert atmosphere.